fix conditionals on the spec after assigning QMAKESPEC

this is of marginal value, as only our own code ever messes with
QMAKESPEC, and we mostly stopped matching on the spec in favor of
compiler and platform flags.

Change-Id: Ibdd9a9c85067623f0f1f064d139d23b4e6b0677d
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

qmake/library/qmakeevaluator.cpp

@@ -946,6 +946,7 @@ QMakeEvaluator::VisitReturn QMakeEvaluator::visitProVariable(
             QString spec = values(varName).first().toQString();
             if (IoUtils::isAbsolutePath(spec)) {
                 m_qmakespec = spec;
+                m_qmakespecName = IoUtils::fileName(m_qmakespec).toString();
                 m_featureRoots = 0;
             }
         }